The free WhatsApp Business app works fine for a single person managing a modest volume of conversations manually.
The moment you need multiple team members responding, automated flows, or to send outside the 24-hour customer-initiated window using approved templates, you need the API.
The API also unlocks click-to-WhatsApp ads with proper attribution and catalog integration — both meaningfully change conversion rates for ecommerce and service businesses.
Setup takes longer (Meta business verification, template approval), but for any business doing real volume, it pays back within weeks.